The present invention relates to treatment of waste water or sewage so as to remove impurities therefrom, in particular to the removal of oxygen demanding impurities and the production of methane therefrom.
Historically there have been a great number of devices and processes for removing impurities from the waste water so as to improve the quality thereof. Such devices have included clarifiers and digesters, and such processes have included both aerobic and anaerobic processes, examples of which are commonly referred to as filter and contact processes. In a typical filter process the waste water is directed upwardly through a filter vessel with microorganisms therein and is thereafter passed through a degasifier and into a settling basin. An underflow from the settling basin is then returned to the filter vessel. In the contact process waste water is injected into a vessel and agitated in contact with sludge containing microorganisms. Effluent leaving the contact vessel is passed through a degasifier and into a settling tank. Heavier sludge falls to the bottom of the settling tank and is recirculated to the contact vessel.
There are a large number of industries which, because of the nature of the product thereof, produce a waste water solution which is very high in organic constituents and is generally unacceptable for disposition in a lake, a river or the like because of the high oxygen demand associated therewith. In particular, environmental laws and regulations are becoming increasingly stringent in requiring the removal of chemical and biological oxygen demanding substances (COD and BOD) from waste water before such water is allowed to flow into public waterways. An example of such a waste water is the effluent from processes wherein molasses or the like is fermented and distilled in the production of alcoholic beverages or ethanol for the use in gasohol, such waste being commonly referred to as slops or mostos. The mostos, which is substantially underflow produced in the distillation process, typically has certain elements such as yeast already removed therefrom, however, same will typically have a very high oxygen demand.
While a number of the prior art processes are in various degrees successful in removing a portion of the oxygen demand from waste water, such as mostos, such processes typically require a degasifier and settler along with an anaerobic vessel with resultant complexities and problems associated with multiple pieces of equipment. Also, prior art processes frequently require both anaerobic and aerobic stages. In many of the prior art processes the methane normally produced during the reduction of oxygen demanding components is lost to the atmosphere.
It has been found that by the addition of certain features to an anaerobic digester, that the ancillary equipment such as degasifiers and clarifiers can be substantially eliminated while increasing the load passing therethrough with substantially equivalent reduction in oxygen demanding components as compared to prior art devices and methods.